Generally, in a boiler using various kinds of coal as fuel, because coal properties, such as indexes indicating the hardness of coal, such as a Hardgrove grindability index (HGI) and moisture percentage, are different, the grindability and transportability in a mill are significantly different. When the feed rate of coal from a coal feeder to the mill is changed for the load fluctuation of the boiler, since coal properties are different, the delay of the coal output from the mill is different depending on the each type of coal. This becomes a disturbance in the control of the steam temperature or, steam pressure in the boiler.
As a method of optimizing the operation of such a boiler, for example, Patent Document 1 (Japanese Patent No. 3746528) discloses a configuration which includes first estimation means which calculates an absorbed heat quantity estimate of a furnace, and second estimation means which calculates an absorbed heat quantity estimate of a final reheater, and which grasps burning characteristics of the boiler on the basis of the ratio of the absorbed heat quantity estimate of the furnace and the absorbed heat quantity estimate of the final reheater. Additionally, Patent Document 2 (Japanese Patent No. 3785088) discloses a configuration adapted to calculate a reference value in the rotation frequency of a rotary classifier according to the feed rate of coal supplied to a coal pulverizer (mill) annexed to a boiler, to add to the reference value a first correction coefficient obtained by normalizing the influence exerted on the control of the rotation frequency and a second correction coefficient obtained from a hardness index value of coal estimated during the operation of the boiler, and to perform the control of the rotation frequency of the rotary classifier on the basis of the output rotation frequency.
Here, a specific example of a conventional control system will be shown below. FIG. 7 is a block diagram illustrating the configuration of a control device including a circuit which calculates a mill coal feed rate command. As shown in this drawing, FX1, FX2, and FX3 are function generators, and preceding signals based on a power generator output command value are input to a changeover switch T. In the changeover switch T, a selection destination is automatically or manually changed over depending on an absorbed heat ratio or an absorbed heat ratio estimation signal. An imperfect differential circuit outputs a so-called boiler acceleration signal (BIR), and a selection destination of this signal is also changed over depending on an absorbed heat ratio by the changeover switch T. Three imperfect differential circuits have different gains, time constants, etc. FIG. 7 shows a case of a circulation boiler, and a drum pressure deviation is input to the control system. The control system is, for example, a PID controller, etc. In the case of a through flow boiler, a main steam temperature deviation is changed to the drum pressure deviation, and input to the control system.
On the basis of a mill coal output command calculated here, the control signal of a mill is calculated by a control device shown in FIG. 8. FIG. 8 is a block diagram illustrating the configuration of a conventional control device including a circuit which calculates an MRS rotation frequency command. In this drawing, FX11 is a function generator which gives a preceding signal based on a mill coal feed rate command value. FX12 is a function generator which gives a standard mill current with respect to the mill coal feed rate command value. In the case of coal which is hard to be pulverized, becomes greater than the standard mill current. A deviation is input to a controller. In this case, the controller is, for example, a proportional controller. The sum of the preceding signal and an output signal of the control system becomes an MRS rotation frequency command signal.
Additionally, as another example, FIG. 9 is a block diagram illustrating the configuration of a conventional control device including a circuit which calculates a mill pressurizing device oil pressure setting value. FX21 is a function generator which gives a preceding signal based on a mill coal feed rate command value. FX22 is a function generator which gives a mill roll lift with respect to the mill coal feed rate command value. A deviation is input to a controller. In this case, the controller is, for example, a proportional controller, etc. The sum of the preceding signal and an output signal of the control system becomes a mill pressurizing device oil pressure setting signal.
As described above, since coal properties, such as HGI and moisture percentage, are different in the case of many kinds of coal, the grindability and transportability in a coal pulverizer are significantly different. Additionally, when the coal feed rate was changed for the load fluctuation of a boiler, the delay of coal output from the coal pulverizer became a disturbance in the steam temperature or steam pressure control of the boiler, and stable control could not be performed. Additionally, even in the same kind of coal, HGI and moisture percentage had considerable variations and the same state was observed. Additionally, conventionally, since the control according to the properties of coal could not be performed in real time, stable operation of the boiler was difficult.
[Patent Document 1] Japanese Patent No. 3746528
[Patent Document 2] Japanese Patent No. 3785088